Cardiac arrhythmias are a significant health problem, and atrial fibrillation is a common type of cardiac arrhythmia. Atrial fibrillation is an irregular heart rhythm that originates within the atria or the upper two chambers of the heart. The pulmonary veins, in particular, can be sources of disruptive electrical impulses that cause atrial fibrillation.
Medication is one known manner of treating atrial fibrillation and is intended to maintain a normal sinus rate and/or decrease ventricular response rates. It is also known to implant devices such as atrial pacemakers to treat atrial fibrillation. Other known methods and devices have been developed for creating therapeutic lesions in myocardial tissue, e.g., by use of minimally-invasive surgical methods, to block unwanted electrical impulses that are believed to be a source of atrial fibrillation. In this context, ablation has come to mean the deactivation, or removal of function, rather than the actual removal of tissue. A number of energy sources may be used for creating these “blocking” lesions that are preferably transmural and extend across the entire heart wall.
Formation of lesions may be performed using both endocardial and epicardial devices and techniques. Endocardial procedures are performed from within the heart. Since the endocardium primarily controls myocardial functions, there are inherent advantages to generating lesions by applying an energy source to endocardial surfaces. One known manner of applying energy for this purpose is utilizing radio frequency (RF) catheters. Certain known endocardial ablation devices include expandable balloons, which are inflated with a coolant such as nitrous oxide. Examples of known lesion formation devices, including cryogenic balloon devices for use in endocardial ablation, and their operation, are described in U.S. Patent Application Publication No. 20060084962, U.S. Pat. Nos. 5,261,879; 6,355,029; 6,468,297; 7,081,112 and 7,101,368, the contents of which are incorporated herein by reference.
While known cryo-ablation devices have been used effectively to some degree, they can be improved. In particular, releasing coolant through a central lumen of a supply tube may result in non-uniform coolant distribution within a cooling element or along a surface of a thermally transmissive region of a cyroablation device. More particularly, the concentration of coolant is highest at the outlet of the tube, and while this may be sufficient for purposes of cryo-ablation in a limited surrounding area, there are uneven mass flows and coolant pressures resulting from this configuration that may result in non-uniform distribution of coolant, which may result in non-uniform cooling and cryo-ablation which, in turn, may result in less effective and more time consuming cryo-ablation procedures.
One attempt to address these issues is to utilize a coolant supply tube that includes apertures that are formed through a wall of the supply tube. This allows coolant to be dispersed from multiple locations of a supply tube. However, such configurations may still result in non-uniform coolant distribution.
For example, referring to FIG. 1, a wall 111 of a coolant supply tube 110 that is positioned within a thermally transmissive region 122 of a body or element 125 of a cryo-ablation device 120 may include multiple apertures (five apertures 112a-e are shown) (generally aperture 112) and a central lumen 112f through which coolant 130 flows. The pressure of the coolant 130 that is dispersed through an aperture 112 is highest at the first or most proximal aperture 112a and decreases along the length of the supply tube 110 to a lowest pressure as coolant 130f exits the distal end of the supply tube 110. As a result, in the illustrated example, the pressure (P2) of coolant 130b dispersed through the second aperture 112b is less than the pressure (P1) of coolant 130a dispersed through the first aperture 112a, the pressure (P3) of coolant 130c dispersed through the third aperture 112c is less than the pressure (P2) of coolant 130b dispersed through the second aperture 112b the pressure (P4) of coolant 130d dispersed through the fourth aperture 112d is less than the pressure (P3) of coolant 130c dispersed through the third aperture 112c, the pressure (P5) of coolant 130e dispersed through the fifth aperture 112e is less than the pressure (P4) of coolant 130d dispersed through the fourth aperture 112d, and so on for more coolant 130 dispersed through more distal apertures. Uneven coolant pressure distribution may result in non-uniform fluid mass flows or coolant volumes (V1-V6) and corresponding uneven cooling and tissue ablation along the thermally transmissive region 122.